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Letter
pubs.acs.org/NanoLett
Plasmonic Nanopore for Electrical Profiling of Optical Intensity
Landscapes
Magnus P. Jonsson and Cees Dekker*
Department of Bionanoscience, Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The
Netherlands
S Supporting Information
*
ABSTRACT: We present a novel method for sensitive mapping of
optical intensity distributions at subdiffraction-limited resolution.
This is achieved with a novel device, a plasmonic nanopore, which
combines a plasmonic bowtie nanoantenna with a 10 nm-indiameter solid-state nanopore. Variations in the local optical
intensity modulate the plasmonic heating, which we measure
electrically through changes in the ionic conductance of the
nanopore. We demonstrate the method by profiling the focal
volume of a 10 mW laser beam that is tightly focused by a highnumerical-aperture microscope objective. The results show a
complex three-dimensional intensity distribution that closely
matches predictions obtained by theoretical calculations of the
optical system. In addition to laser profiling, the ionic conductance
of a nanopore is also shown to provide quantitative estimates of the temperature in the proximity of single plasmonic
nanostructures.
KEYWORDS: Nanopore, optical nanoantenna, plasmonic heating, optical profiling, nanoplasmonics, thermoplasmonics
T
technique suitable for profiling of high-intensity lasers, but of
limited use for mapping low-intensity light beams.
Here, we present a novel concept for profiling of lowintensity optical field distributions at subdiffraction limited
resolution. The method is based on a plasmonic nanopore,
which combines a single plasmonic gold bowtie nanoantenna
positioned on a thin solid-state membrane and a nanopore that
is drilled through the membrane at the center of the bowtie
antenna (see Figure 1). The plasmonic antenna, consisting of
two gold nanotriangles separated by a 10 nm gap, acts as a
nanoscale detector that converts local optical intensity
variations into temperature variations. These temperature
variations are measured electrically using the nanopore as a
local temperature probe, thereby utilizing the temperature
dependence of the nanopore’s ionic conductance.11
Metal nanostructures are ideal nanoscale light-to-heat
converters, which is related to their strong light absorption
via excitation of collective oscillations of the electrons in the
metal. These oscillations are known as localized surface
plasmons, and the process results in dissipation of heat to the
local environment.12 There is currently a strong interest in
using the thermoplasmonic properties of metal nanostructures
for a wide variety of applications, ranging from plasmonassisted nanochemistry13 to photothermal therapy14,15 and
here is a strong interest in the development of new types
of lenses for bright-field super-resolution microscopy and
microscopy without aberrations. Promising approaches utilize
negative refraction and metamaterials,1,2 nanostructured superoscillatory lenses,3 and phase discontinuities in arrays of optical
nanoantennas.4,5 The precise characterization of optical systems
resulting from advances in these areas poses an urgent need for
methods that can accurately map three-dimensional optical field
distributions with high resolution and a high signal-to-noise
ratio. For low-numerical aperture (NA) lenses, for which
subdiffraction-limited resolution is not required, the intensity
profiles can be mapped using a macroscale pinhole and a light
detector.4 Near-field optical scanning microscopy has been used
to improve the resolution to the subdiffraction limited regime
and was, for example, used to map the intensity profile of light
focused by a lens with NA 0.4 at 100 nm resolution.6 Other
concepts utilize scattered or emitted light from single metallic
particles or fluorescent beads, which can be scanned through a
focal volume to map out the intensity distribution.7−10
Recently, Keyser et al. used a solid-state nanopore device to
map the increase in temperature of a liquid caused by heating
by a focused high-power infrared laser beam.11 The temperature distribution could be converted to the intensity
distribution of the laser using the heat equation. The technique
can provide few-nanometer resolution, although with a signalto-noise ratio that is rather limited by the low absorption
coefficient of the aqueous buffer medium. This makes the
© 2013 American Chemical Society
Received: November 14, 2012
Revised: February 8, 2013
Published: February 12, 2013
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our notation, the beam propagates in the positive direction
along the z-axis, and it is polarized along the y-axis. Unless
stated otherwise, the long axis of the plasmonic bowtie probe
was oriented parallel to the polarization axis, as indicated in
Figure 1b (longitudinal mode).
Results obtained by measuring the nanopore current while
scanning the plasmonic nanopore along the focal plane of the
beam are shown in Figure 2a, where the inset shows a smaller
Figure 1. The principle of plasmonic nanopore optical profiling. (a)
Schematic representation of the concept. Variations in the local optical
intensity within a focal volume modulate the heating of the
nanoplasmonic structure. These temperature changes are measured
electrically through changes in the ionic current flowing through the
nanopore. Optical intensity distributions are obtained by scanning the
plasmonic nanopore in three dimensions using a piezoelectric stage.
(b) Three-dimensional schematic representation of the plasmonic
nanopore probe, consisting of a plasmonic bowtie nanoantenna with a
nanopore in the gap. The double-sided arrow indicates the polarization
that was used in most measurements (the longitudinal mode). (c)
TEM image of a typical plasmonic nanopore. The inset shows a
zoomed image of the 10 nm nanopore (false colored) in the gap
between the two gold triangles.
Figure 2. Ionic current maps obtained by scanning a plasmonic
nanopore through the focal volume of a focused laser beam. (a)
Experimental 2D scan through the laser focal plane, obtained using 50
nm step size of the piezo stage and a 200 mV voltage applied over the
nanopore. The colormap shows the measured current in nA. The inset
in the middle (the white box) shows a higher resolution scan of a
smaller region (at 10 nm step size) acquired using a 100 mV voltage
applied over the nanopore (the colormap of the inset was modified
accordingly). (b) Data of panel (a) presented as a 3D image. The
current is denoted I. (c) Normalized line scans in y through the focus
position, using the probe in the longitudinal mode (blue plus marks)
and in the transverse mode (red crosses). The dashed black line shows
the theoretical profile obtained by vectorial calculations. (d)
Normalized line scans along z through the focus position, using the
probe in the longitudinal mode (blue plus marks) and in the transverse
mode (red crosses). The presented values in panels (c) and (d) were
normalized using the highest and lowest values of each scan, because
the absolute current changes were lower for the transverse mode than
for the longitudinal mode. This is in agreement with the fact that the
plasmon resonance of the transverse mode was not designed to match
the laser wavelength (see Figure S1), and thus, the absorption and
associated temperature increase is lower for the transverse mode.
imaging.16 Here, we investigate the local heating in a plasmonic
nanopore and present the first use of such plasmonic heating
for profiling of optical intensity landscapes.
Fabrication of Plasmonic Nanopore Devices. Thermoplasmonic nanopore probes were made from silicon (Si) chips
containing thin (20 nm) freestanding silicon nitride (SiN)
membranes. We fabricated arrays of bowtie nanoantennas on
top of these membranes using electron beam lithography, as
detailed in the Methods section. Based on finite-difference time
domain (FDTD) simulations (see the Supporting Information), we designed the bowties to have their plasmon resonance
matching our laser wavelength (785 nm) when excited with
light polarized along the long side of the structure. We denote
this orientation as the longitudinal mode (see illustration in
Figure 1b) and the perpendicular orientation (rotated 90°
around the optical axis) as the transverse mode. Each bowtie
antenna consisted of two 30 nm thick equilateral gold triangles
with a length of around 60 nm from the tip to the opposite flat
side of the triangle. In the final process step, a transmission
electron microscope (TEM) was used to locate a bowtie with
an approximately 10 nm gap between the triangles, followed by
drilling a single 10 nm-in-diameter pore through the SiN
membrane at the location of the gap. Figure 1c shows a TEM
image of a final structure. For pore conductance measurements,
the nanopore chip was placed in a flow cell such that the
membrane separates two buffer compartments (see Figure 1a).
The ionic current through the nanopore was measured using a
commercial patch clamp amplifier, as detailed in the Methods
section.
High-Resolution Optical Intensity Mapping with a
Plasmonic Nanopore. We first demonstrate our method by
profiling the focal volume of a 10 mW 785 nm laser that is
tightly focused through a microscope objective with NA 1.2. In
region obtained with 10 nm step size. Figure 2b is a threedimensional representation of the same measurement and
demonstrates that the method is able to provide information on
minute intensity changes, such as the surrounding airy disk
pattern. It is also possible to discern a small asymmetry
between x and y. This asymmetry remained on the same
absolute axis when the polarization was rotated 90°, indicating
that it is dominated by residual imperfections in the optical
alignment. At the position of maximum intensity of the laser
beam, the current almost doubles compared with the baseline
value. We can estimate the local temperature increase (ΔT)
induced by the plasmonic heating from the relative change in
current (ΔI/I) as ΔT = (32.9 °C + T1) × ΔI/I (see the
Supporting Information), where T1 is the initial temperature in
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degrees Celsius (22.5 °C in our temperature-controlled
laboratory). Using this approach, a doubling in conductance
corresponds to a temperature increase of around 55 °C. Hence,
at 10 mW the maximum temperature of the buffer is around 78
°C in the experiment of Figure 2a,b.
A possible influence of plasmon excitation mode and probe
geometry on the results was investigated by measuring the
intensity distribution in the longitudinal mode, then rotating
the plasmonic nanopore probe 90° around the optical axis and
profiling the same source again, thus using the transverse mode
of the optical nanoantenna. Results obtained for the two
different probe orientations are shown in Figure 2c for scans
along the polarization direction through the focal plane (see
Figure S3 for the same data presented also on a logarithmic
scale). It is clear that the measured profiles for the different
probe orientations overlap very well. This indicates that the
method is insensitive to probe geometry and excitation mode.
Furthermore, this implies that the method does not require a
perfect match between the illumination wavelength and the
plasmon resonance and also that the process is insensitive to
the detailed spatial electromagnetic field distribution, which is
different for the two different excitation modes. With respect to
the latter we note that, regardless of the electromagnetic field
distribution, rapid thermal diffusion will result in a relatively
uniform temperature distribution over the plasmonic nanostructure.17
Importantly, the measured profiles also match the theoretically predicted profile (dashed black line in Figure 2c, see
details below). The full widths at half-maximum (fwhm) for the
profiles in Figure 2c are 554 nm, 537 nm, and 530 nm, for the
longitudinal mode, the transverse mode, and the theoretical
calculation, respectively. Hence, possible convolution effects
due to the finite probe size are minor in our experiments, which
is in agreement with the subdiffraction-limit dimensions of the
plasmonic pore. As shown in Figure 2d, the normalized profile
obtained through the center of the beam along the optical axis
(z) is also not significantly influenced by the probe orientation.
The results indicate that the concept can be applied also to
other structures than bowtie nanoantennas. The fact that the
profile obtained along the optical axis (Figure 2d) is not a
simple Gaussian, but contains multiple peaks and dips, is in
agreement with theoretical predictions of the optical system, as
investigated in detail below.
Comparing Experimental Results with Theoretical
Calculations. Next, we investigate the accuracy of the
presented method by comparing experimentally obtained
intensity profiles with calculated intensity maps of the optical
system. A laser beam that is tightly focused by a microscope
objective is ideal for this purpose, because it provides a wellknown and elaborate intensity pattern.6,18,19 We profiled this
system using our thermoplasmonic nanopore approach, and a
high-resolution y-z current scan is presented in Figure 3a. First
note that the pattern is highly nonsymmetric along the optical
axis. This is due to aberrations, in our case primarily due to a
difference between the cover slide thickness (measured to be
146 μm) and the optimum thickness for the microscope
objective (210 μm).18 We used vectorial theory based on the
approach presented by Richards and Wolf20 to construct a
theoretical intensity map of the optical system.18−20 The
introduction of a so-called apodization function, according to
the notation of Woehl et al.,18 allowed us to set the beam waist
equal to the aperture of the microscope objective (filling factor
= 1). The detailed parameters that were used for the
Figure 3. Measured and theoretical intensity distributions of a 785 nm
laser beam focused into a flow cell via a microscope objective (NA =
1.2). The beam is polarized along the y-axis and propagates along the
z-axis in the positive direction. (a) Current distribution measured for a
y−z scan using a step size of 50 nm. (b) Calculated intensity
distribution of the same optical system.
calculations can be found in the Supporting Information. The
calculated intensity map is presented in Figure 3b, and it is clear
that the experimental results closely match the theoretical
calculations. No fitting parameters were used, and the minor
differences between the experimental results and the calculations are likely due to small differences between the settings
used in the calculations and the actual parameters in the
experimental situation.
We thus present a new method for mapping optical intensity
landscapes based on a single plasmonic nanopore. The
plasmonic nanopore acts as a local light detector by converting
local light intensity to heat, which, in turn, is converted to an
electrical signal via the ionic conductance of the nanopore. The
method is very sensitive due to the strong light absorption of
the plasmonic nanoantenna, and it provides a high signal-tonoise ratio at subdiffraction-limited resolution already for low
intensity laser beams. The potential of the method is
demonstrated by an excellent correspondence between
experimental results and theoretical calculations. The high
dynamic range and signal-to-noise ratio is due to the fact that
the total signals (∼10 nA at 10 mW laser power) are 3−4
orders of magnitude higher than the noise level of typical
nanopores (∼1−10 pA at an acquisition rate of around 10
Hz).21 The ∼100 nm dimensions of the nanoantenna provide
subdiffraction-limited spatial resolution. The resolution limit
will be evaluated in more detail in future work and may, for
example, be further improved by the use of even smaller
plasmonic nanoantennas. In this respect we note that, in
contrast to light scattering-based methods, our method is based
on absorption and will not suffer from the fact that absorption
becomes increasingly dominant over scattering as the size of the
plasmonic nanostructure decreases.22
As mentioned in the introduction, our group previously used
conventional (nonplasmonic) nanopores for optical profiling.11
That method was based on light being absorbed by the buffer
medium, which for an intense focused laser beam will result in
significant heating in the entire focal region. The resulting
spatial temperature distribution could be mapped by the
nanopore and, in turn, converted to the laser intensity profile
through the heat equation. We would like to stress that our new
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More detailed information on nanopore fabrication using TEM
can be found elsewhere.29,30
Profiling Experiments. After a short oxygen plasma
treatment, the plasmonic nanopore chip was mounted in a
flow cell such that it separates two compartments containing a
buffer solution (1 M KCl, 10 mM TRIS, 1 mM EDTA, pH 8).
The ionic current flowing through the nanopore was measured
using a patch clamp amplifier (Axon Axopatch 200B, Molecular
Devices, US) connected to a computer via a DAC card (USB6251, National Instruments, US). Each buffer compartment was
connected via an agarose gel salt bridge to a separate
compartment containing potassium ferricyanide, potassium
ferrocyanide, and 1 M KCl. The ionic current was then picked
up using platinum wires dipped in these compartments. For
scanning measurements, the flow cell was attached to the piezo
stage (P-545, Physik Instrumente, Germany) of a custom-built
inverted microscope. Light from a 785 nm diode laser
(Omicron-Laserage Laserprodukte GmbH, Germany) was
expanded to about 7 mm using two lenses and focused into
the flow cell through a water-immersion objective (60×, NA
1.2, Olympus, The Netherlands). Profiling measurements were
performed using a custom-designed LabView program (National Instruments, US) that controlled our instruments.
method is based on an entirely different concept, as it does not
require such conversion, but instead directly provides a measure
of the light intensity distribution. The fundamental difference
between the two approaches is that our new method is not
based on light absorption by the buffer, but on local light
absorption by the nanopore itself through excitation of
plasmons in the nanoantenna. Due to the strong light−
nanoantenna interaction, the signals obtained using our
plasmonic nanopore are also almost 2 orders of magnitude
higher compared to the signals obtained using a nanopore
without plasmonic nanoantenna, making our method particularly suitable for profiling of low-intensity light beams.
Future work may take the opportunity to investigate
plasmonic nanopore systems based on other geometries than
the bowties presented in this work. For example, plasmonic
antennas can be purposely designed to have strongly
polarization-dependent excitations, which may be used to
independently probe the individual contributions from different
polarizations to the intensity in a focal volume. Systems with
circular symmetry, such as nanorings,23 may instead be useful
for probing the total intensity in nonpolarized beams.
Apart from providing a new concept for profiling optical
intensity landscapes, this study is also the first report
demonstrating that a nanopore can be used as a probe to
investigate heating effects in single plasmonic nanostructures.
We demonstrated a local temperature increase of 55 °C upon
illumination of our plasmonic structure with a 10 mW focused
laser source. We foresee the use of the principle to investigate
and compare the thermoplasmonic properties of different
nanoplasmonic structures. Furthermore, for a given plasmonic
nanopore and excitation intensity, the plasmonic heating at a
given wavelength is expected to be sensitive to shifts in the
plasmon resonance, as, for example, induced by biomolecular
binding reactions. Hence, the principle of using the ionic
conductance of a nanopore to probe changes in plasmonic
heating may also allow for electrical read-out in plasmonic
nanopore sensors, which have shown highly suitable for
biosensing, but typically rely on optical spectroscopy.24−27
Methods. Sample Fabrication. The plasmonic nanopore
chips were made starting from prefabricated wafers containing
256 individual chips with freestanding SiN membranes (65 μm
× 65 μm and 20 nm thick). The wafer was spin-coated with a
thin layer of poly(methyl methacrylate) (PMMA, 950 K, 2%
dilution in anisol) and baked on a hot plate at 175 °C for one
hour. Next, the wafer was diced into smaller pieces (4 × 4
chips) and patterned with an electron beam (2000 μC/cm2,
EBPG5000Plus HR 100, Vistec, Germany). After development
in a mixture of MIBK and IPA (1:3, 60 s), a short oxygen
plasma step was used to remove possible resist residues on the
SiN surface. One nm titanium and 30 nm gold was then
deposited using electron-beam-assisted evaporation. The
titanium layer was added to improve the adhesion of the
nanostructures to the SiN membrane. The final plasmonic
bowtie structures were obtained by PMMA lift off (positive
photoresist remover, 60 °C, Baker PRS 3000). Similar
protocols have previously been used to fabricate plasmonic
nanostructures on thin membranes.28 After dicing into
individual chips, the electron beam of a TEM (Philips
CM300UT-FEG operated at 200 kV, with a ∼10 nA beam
current and an around 10 nm beam diameter) was used to drill
a single 10 nm-in-diameter pore through the SiN membrane
right at the gap position of a suitable plasmonic bowtie antenna.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information contains results from FDTD
simulations of the plasmonic nanoantenna, details on the
relation between ionic pore conductance and temperature,
profiling results presented on a logarithmic scale, and details on
the vectorial calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We would like to thank Meng-Yue Wu for TEM drilling and
imaging, Daniel Burnham for vectorial theory code, Samet
Mercan for help with preliminary measurements, Jelle van der
Does for help in construction of the setup, and Jakob
Kerssemakers, Xander Janssen, Gautam Soni, and Iwijn de
Vlaminck for fruitful discussions. The work was financially
supported by the Wenner-Gren Foundations and the ERC
project NANOforBIO.
■
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